i think that you must return x[] to then main() function
see my code below.

Code:

#include <stdio.h>
#include <stdlib.h>

char * squeez(char [],char []);

char * squeez(char s[],char z[]){int i,j,found,count=0;char *x=NULL;for (i=0;s[i];i++){
found=1;//we suppose that this char-->s[i] is not in second stringfor (j=0;z[j];j++){if (s[i]==z[j]){//if this char-->s[i] exists in second string then we supposed wrong!
found=0;//this char-->s[i] is not good because it exists in second string break;//we found a wrong value so stop searching the rest of second string for s[i]
}
}if (found==1){//yes we found one that not exists in second string
count++;//increase counter
x=(char *) realloc(x,count*sizeof(char)); //alocate memory for it
x[count-1]=s[i]; //store it in x[]
}

1 Remember = and == are not the same.In your if statement you use = this would assign the variable a value whereas == checks if two values are equal.
2 When setting a flag dummy try to use 1 for on and 0 for off this makes it easier to understand don't use any arbitrary value
3 When passing arrays try to use pointers.It's much more easier